A 2-week-old infant, born at term to healthy, non-consanguineous parents, presents with severe and persistent unconjugated hyperbilirubinemia with a peak level of 28 mg/dL. There is no evidence of hemolysis. The infant has required multiple exchange transfusions and continuous phototherapy. A therapeutic trial of phenobarbital for one week results in no decrease in serum bilirubin levels. What is the most likely diagnosis?